This invention relates generally to electrically conductive materials adapted to operate in hostile environments and is particularly directed to a zirconia-based material which is tough, highly resistant to fracture due to thermal and mechanical stress, and is conductive.
Electrically conductive materials are frequently used in hostile environments. For example, a process may only be carried out at elevated temperatures requiring any associated electrical conductors to operate at these high temperatures. High operating temperatures impose demands upon the physical structure and mechanical characteristics of the conductor. For example, temperature extremes frequently result in cracking, or other forms of deterioration, of the conductor or cause unwanted changes in the fracture stress or hardness and/or fracture toughness of the material. Extreme temperature fluctuations also place severe demands upon the electrical conductor frequently resulting in its cracking, increased susceptibility to corrosion, and general physical deterioration. Where the conductor is metal-based, prior art solutions frequently called for the addition of various combinations of alloys having desired thermal and electrical characteristics to provide the conductor with the desired characteristics. However, the lack of stability of the metal-based conductors limits their use at high temperatures.
Because of the aforementioned limitations of metal-based conductors, increasing effort has gone into the development of ceramic conductors. Ceramic-based conductors are finding increased use in such applications as solid oxide fuel cells which are characterized as operating between 700.degree. and 1100.degree. C. In these fuel cells, hydrogen or a high order hydrocarbon is used as the fuel and oxygen or air is used as the oxidant. The fuel cell is generally comprised of a layered structure including an anode, a cathode and an electrolyte disposed therebetween. The electrolyte insulates the cathode and anode from one another with respect to electron flow, but permits oxygen ions to flow from the cathode to the anode. The electrolyte must therefore exhibit high conductivity at these elevated temperatures while being capable of supporting the anode and cathode in a configuration which facilitates the formation of channels through which the oxygen and hydrogen may easily flow. Large cracks in the electrolyte allow fuel and air to mix and reduce cell performance.
The present invention overcomes the aforementioned limitations of the prior art by providing a tough conductive material, without adversely affecting its conductivity at high operating temperatures. The ceramic-based conductive material of the present invention is particularly adapted for use as the electrolyte in a solid oxide fuel cell or in an electrochemical sensor for combustion exhaust, but is not limited to these applications and can be used in virtually any situation where an electrical conductor is required to operate at elevated temperatures or is subjected to large fluctuations in operating temperature.